The MAC0FXCHM or const_2d_chm_Fx data product is the MERRA Data Assimilation System 2-Dimensional Constants at native Fv resolution. MERRA, or the Modern Era Retrospective-analysis for Research and Application, is a NASA reanalysis for the satellite era (1979-2016) using the Goddard Earth Observing System Data Assimilation System Version 5 (GEOS-5 DAS). MERRA Science Data and Data Processing Questions Do you have a question about MERRA? Take a look at the File Specification Document and if that doesn't answer your question, users can contact staff with questions on the data, data processing, and science. Send questions to merra-questions@lists.nasa.gov.